1. Field
The present invention relates to the field of computer memories, and more particularly, to improved apparatus and methods for storing and transmitting data representative of images to a display system.
2. Art Background
In many computer systems, it is quite common to represent and convey information to a user through digital images. These images may take a variety of forms, such as for example, alphanumeric characters, cartesian graphs, and other pictorial representations. In many applications, the digital images are conveyed to a user on a display device, such as a raster scan video monitor, printer or the like. Typically, the images to be displayed are stored in digital form, manipulated, and then displayed.
In many computer display systems, data in the form of binary quantities representative of picture elements comprising an image on a display are stored in a memory referred to as a "frame buffer", such that each data bit (a 1 or 0) is mapped onto a corresponding picture element ("pixel") on the display. Memories used to store representations of each pixel comprising an image are known as "bit-map memories". Thus, there is a one-to-one correspondence between data contained in the memory and the image displayed. A number of bit-maps may be defined within the memory such that color may be associated with each bit-map, thereby permitting multi-colored images to be displayed on an appropriate color monitor or the like. The generation and manipulation of a digital image requires that a large number of bits in the bit-map be updated after a modification.
A number of display systems utilize "dual-ported" memory devices as frame buffers which permit a display processor to read data comprising an image being displayed in order to permit the data currently stored within the dual-ported memory to be updated. The display processor is often required to first read the data from the dual-ported memory device, and then internally modify the data to form an appropriate binary representation of the new image to be displayed. This updated data must then be written back into the dual-ported memory such that it may be accessed through another memory port by the particular display device for subsequent display.
It has been found that the use of a dual-ported memory display system significantly reduces system performance, inasmuch as data may not be updated by the display processor while the display device is reading the contents of the bit-mapped memory for display (the process of reading the contents is typically called a "refresh" cycle). In addition, the display processor must often read data stored within the dual-ported memory frame buffer, modify the data, and then write the data back into the memory. The requirement of a read and write cycle by the display processor in conjunction with the necessity for the execution of a refresh cycle by the display device, results in lower overall speed when updating and generating images for display.
One factor limiting the speed at which an image represented in a bit-map is manipulated is the cycle time of the memory devices comprising the memory. Typically, each memory device represents blocks of adjacent pixels, or other display elements, defining the display. Thus, a digital image such as for example, a line ("vector") will likely be represented by a plurality of pixels the states of which are stored in memory devices representing one portion of the entire bit-map. Accordingly, in application requiring high speed graphic image manipulation, such as animation, the speed at which the computer system is capable of updating and displaying digital images is dependent upon the cycle time of the memory devices. Memory devices, such as dynamic random access memories (D-RAMS), have cycle times of approximately several hundred nanoseconds. Thus, in systems where the computer or display processor is capable of higher speed data manipulations than the display memory devices, the overall system performance is constrained by the limiting cycle times of the memory devices comprising the frame buffer.
As will be described, the present invention provides apparatus and methods for efficiently modifying data comprising an image, and transferring the data to a frame buffer for display on a display system. The present invention thereby permits the modification and updating of images by a display processor at high speed, and avoids the delays associated with dual-ported memory display systems known in the prior art.